1. Field of the Invention
The present invention relates to an acceleration sensor having a piezoelectric element, and more particularly to an acceleration sensor used for an air bag installed in a vehicle.
2. Description of the Related Art
An acceleration sensor employing a piezoelectric element has been used for an air bag installed in an automobile in order to detect an acceleration caused by an impact. Acceleration sensors having various kind of structures have been proposed for this application, and an acceleration sensor using a bimorph type piezoelectric element has been known as a high sensitivity type as disclosed in Japanese Patent Laid-open Publication Nos. 6-273439, 6-324073, etc.
As shown in FIG. 1, a conventional acceleration sensor 1 using a bimorph type piezoelectric element has a bimorph type piezoelectric element 2 and insulating case 3. The acceleration sensor 1 is mounted on a substrate 4.
The bimorph type piezoelectric element 2 has a structure in which a first piezoelectric ceramic plate 5 and a second piezoelectric ceramic plate 6 are laminated. A first signal electrode 7 and a second signal electrode 8 are formed on outer main faces of the first piezoelectric ceramic plate 5 and the second piezoelectric plate 6, respectively. The first signal electrode 7 and the second signal electrode 8 overlap through the piezoelectric ceramic plates 5 and 6 in the central portion along the longitudinal direction of the piezoelectric element 2. An intermediate electrode 9 is formed between the first piezoelectric ceramic plate 5 and the second piezoelectric ceramic plate 6 such that the intermediate electrode 9 is located between the overlap portions of the first signal electrode 7 and the second signal electrode 8.
The first piezoelectric ceramic plate 5 and the second piezoelectric ceramic plate 6 are uniformly polarized in opposing directions with respect to each other. The first signal electrode 7 extends toward one of two opposing ends of the piezoelectric element 2 along the longitudinal direction thereof while the second signal electrode 8 extends towards the other end of the piezoelectric element 2 along the longitudinal direction thereof.
The insulating case 3 comprises a first clip portion 10, a second clip portion 11, and substrates 12 and 13. The clip portions 10 and 11 have recesses 10a and 11a, respectively and clip the piezoelectric element 2 at the both ends of the longitudinal direction thereof, thereby holding the piezoelectric element 2 at the both ends of the longitudinal direction thereof while allowing the piezoelectric element 2 to vibrate. The substrates 12 and 13 have a recess 12a and a recess (not shown), respectively and clip the piezoelectric element 2 in a direction perpendicular to a direction in which the clip portions 10 and 11 are faced.
The bimorph type piezoelectric element 2 has a good sensitivity to an acceleration along the thickness direction of the piezoelectric element 2 and no sensitivity against an acceleration along the direction indicated by an arrow Q.
Although the bimorph type piezoelectric element 2 outputs a reasonable sensing potential, the sensing charge is small due to a small static capacitance of the bimorph type piezoelectric element 2. Therefore, when the sensing output is amplified using a charge amplifier, the signal to noise (SN) ratio degrades.
Moreover, there arises a problem that a low frequency component of the signal detected by the bimorph type piezoelectric element 2 cannot be measured since a high pass filter created between the charge amplifier and the bimorph type piezoelectric element 2 has a high cutoff frequency at a lower frequency side. In the case where the impedance of the charge amplifier is increased so as to measure a low frequency component, the noise on the detected signal might be such increased that the SN ratio degrades.
It is possible to increase the static capacitance by making the piezoelectric element 2 thinner in thickness or larger in width. However, in the case of making the piezoelectric element 2 larger in width, this results in the acceleration sensor having a large size, which would be adverse to the commercial demand. On the other hand, in the case of making the piezoelectric element 2 thinner, the mechanical strength of the piezoelectric element 2 degrades, which may decrease the production yield and increase the possibility of destruction due to a large acceleration.
For the foregoing reasons, there is a need for an acceleration sensor which has a high charge sensitivity and a large static capacitance as well as a small size and an appropriate mechanical strength.
The present invention provide an acceleration sensor that satisfies this need. The acceleration sensor comprises a piezoelectric element having at least three piezoelectric layers, a plurality of first electrodes and a plurality of second electrodes. The piezoelectric layers are stacked with each other in a thickness direction so as to form a laminate having first and second opposed ends in a lengthwise direction thereof. The first and second electrodes are provided alternately between the piezoelectric layers and on top and bottom surfaces of the laminate such that the first electrodes extend from the first end toward the second end and the second electrodes extend from the second end toward the first end and partially overlap with each other via the piezoelectric layers. The acceleration sensor further comprises a support for holding the piezoelectric element at the vicinity of the first and second ends of the laminate. The piezoelectric layers are polarized in the thickness direction such that charges having opposite polarities are accumulated at the first and second electrodes, respectively, when the piezoelectric element receives an impact caused by an acceleration.
The piezoelectric layers may be polarized at the region where the first and second electrodes overlap.
In one embodiment, at least one of the stacked piezoelectric layers is not polarized.
In another embodiment, the piezoelectric element has an even number of the piezoelectric layers, greater than three, adjacent ones of the piezoelectric layers, except a pair of the piezoelectric layers which are located at the middle of the laminate, are polarized in opposite directions with each other and the pair of piezoelectric layers are polarized in the same direction.
In accordance with another embodiment, the piezoelectric element has an odd number, greater than two, of piezoelectric layers, all of the piezoelectric layers, except one located at the middle of said piezoelectric element, being polarized.
In accordance with another embodiment, the acceleration sensor includes a substrate upon which the support is mounted, the support holding the laminate at an angle of 90xc2x0 with respect to the substrate. Advantageously, the angle is between 0xc2x0 and 90xc2x0 and, preferably, it is equal to or less than 45xc2x0.
According to the preferred embodiments of the invention, positive and negative charges induced in each of the piezoelectric layers are effectively accumulated at the first electrodes and the second electrodes without cancellation. This structure also provides the acceleration sensor with a large static capacitance. Therefore, piezoelectric type acceleration sensor which has a high charge sensitivity, detects an acceleration having a low frequency component and is small in size can be realized.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following detailed description in conjunction with the accompanying drawings.